In my production environment, I have a mostly working Oracle 12c Cloud Control environment, managing several database instances. On all of the databases, I am unable to use the Compare period ADDM feature ( Instance > Performance > AWR > Compare Period ADDM).

When I select that menu option, I see the message "To be able to use this feature some PL/SQL packages need to be loaded into the target database's monitoring schema, DBSNMP." I have been searching for further information in the specific packages that need to be loaded per the message, but neither Orafaq, support.oracle.com nor Google seem to have those details.

Was some time way back, but when that message appeared I remember it contained some link that when clicked it installed whatever it needed.
Try: Enterprise -> Quality Management -> Data Discovery and Modeling (it also needs the procedures).
Good luck!